1. Field of the Invention
The present invention relates to the use of low structure carbon blacks in curable coating compositions, curable coatings, and cured coatings comprising carbon blacks, and black matrices that can be formed therefrom.
2. Description of the Related Art
Black matrix is a generic name for materials used in color displays to improve the contrast of an image by separating individual color pixels. In liquid crystal displays (LCDs), the black matrix is a thin film having high light-shielding capability and is formed between the three color elements of a color filter. In LCD's using thin film transistors (TFT), the black matrix also prevents the formation of photo-induced currents due to reflected light in the TFT.
The black matrix layer in liquid crystal displays has been manufactured by vapor deposition of Cr/CrO. Although chromium based films have excellent light-shielding capabilities, the metal vapor deposition process is expensive. In addition, chromium use and disposal is subject to increasingly restrictive environmental regulations. Chromium films also have low resistivity, which restricts the electrical design of LCDs to a subset of the possible design configurations.
Black pigments such as carbon black have been used in polymer compositions to make resistive black matrices. The structure and surface area of these blacks are chosen to permit a particular loading level of carbon black in a matrix and to reduce conductivity and charge accumulation in the media. Increased loading level increases the optical density (OD), a measure of the opacity of a material, of the media but also increases the viscosity of the coating compositions used to produce the media. Thus, it is often difficult to achieve the desired balance of overall properties. For example, while a black matrix containing a carbon black pigment could provide the required light-shielding capabilities (that is, a threshold optical density), the film might have only a modest resistivity, limiting its ability to inhibit photo-induced currents. Alternatively, if a highly resistive film were produced, the OD might be too low to be commercially viable.
Decreasing the structure of the component carbon black can decrease viscosity, allowing thinner layers of the media to be deposited without defects, or it can allow more carbon black to be incorporated at a given viscosity, resulting in a higher optical density. One method of controlling the structure of a furnace carbon black is by adding alkali or alkaline metal elements to a furnace while burning a carbonaceous feedstock. However, the resulting metal component in the carbon black can contribute to increased conductivity, and non-carbon materials in the media do not contribute to optical density. Furthermore, as potassium and other metal elements are added to the furnace, the resulting black has more charged groups on the surface and is thus more hydrophilic. Thus, a decrease in structure may come at the expense of increased conductivity and decreased optical density resulting from non-carbon elements in the carbon black. More hydrophilic or acidic blacks (e.g., pH less than 6) may not be compatible with as wide a range of polymers and other components that would be otherwise desirable for use in coating or printing applications. In addition, while optical density or tint may be increased by increasing surface area, it becomes increasingly difficult to decrease the DBP as the surface area is increased. Thus, it is desirable to develop carbon blacks having low structure but also having low amounts of alkali metals and high hydrophobicity and that do not compromise electrical properties, optical density and viscosity in compositions and devices incorporating the carbon black.